Muscles – Thorax Muscles and Thoracic Wall

The complexity of the musculoskeletal system is often a big issue for medical students. To learn about muscles effectively, a clear and logical grouping into systems with unique structures is needed. In this article, in addition to detailed descriptions of the origin, insertion, action and innervation, medical students will obtain an overview in tabular form for visualization of the muscle groups. Attention: Due to conflicting information in primary literature about the muscles' origins and insertions, it is advisable to always consult the latest copies for studying.

00:00
Now, we’ll shift our attention to the muscles
of the thorax and when we look at musclesof the thorax, we can divide them or assign
them into one of two categories. We have extrinsicmuscles and we have intrinsic muscles. Extrinsic
muscles will have an attachment to the thoracicwall, but then another attachment that lies
outside of the thoracic wall such as the clavicleor the humerus. Intrinsic muscles will attach
to the components of the thoracic wall, sternumto ribs, ribs to ribs, for example.
Let’s focus first, then, on the extrinsicmuscles of the thorax. There are just four
of them that we’ll work through, the pectoralismajor, the pectoralis minor, the serratus
anterior and lastly, the subclavius. Two ofthese four muscles are depicted on this particular
illustration. We have the pectoralis majorwhich is responsible for providing the major
contour to the anterior thoracic wall andwe can also see these interdigitating muscle
slips on the more lateral aspects of the walland these belong to the serratus anterior.
We will now spend a brief moment by lookingat each one of these extrinsic muscles. The
first one to take a look at is the pectoralismajor. We want to understand its attachments,
its innervation. And then we’ll also wantyou to understand what it does when it shortens,
what are the functions or actions of thisparticular muscle.
The pectoralis major is shown right in throughhere. Very prominent muscle, again, on the
anterior thoracic wall. It is described ashaving two heads, a clavicular head because
of its attachment to the medial third of theclavicle. Also has a sternocostal head, which
is the major head, because of its attachmentto the sternum and to the costal cartilages
of ribs 2 through 7. It also has an attachmentto the aponeurosis of the external abdominal
oblique. Its distal attachment or its insertionis on the medial crest of the intertubercular
groove or sulcus of the humerus. And so, thispoints of wide origin or proximal attachments,
all of a sudden, will converge to have a veryspecific point of insertion or distal attachment
to that medial breast. This muscle is innervatedby the medial and lateral pectoral nerves.
And now, what does it do when it shortens?What are its actions? And the best way to
understand the actions of any muscle is topredict what the actions are based on the
attachments. So, if we think about the attachmentsrunning from thoracic wall, clavicle, sternum,
ribs, aponeurosis of the external abdominaloblique, to the humerus, the lateral crest,
some of the actions of the pectoralis majorare as follows. It is a very powerful producer
of adduction of the humerus.
03:29
So, if we start with our arm a little bit
away from the midline of the body and thepectoralis major starts to shorten here on
the right side, it is going to bring yourhumerus back to the midline of your body.
Another function of the humerus is to pullyour arm forward and that is called flexion.
Another function yet of the humerus is tomedially rotate your humerus or have internal
rotation of your humerus. And if your armsare above your head, the pectoralis major
muscle on both sides will help pull the trunkof your body up toward your arms. So, if you’re
in the rock climbing and you have your handsabove your body, the pectoralis major, in
association with latissimus dorsi, willhelp pull your trunk back toward your humeri.
The pectoralis minor lies deep to the pectoralismajor and we can see the pectoralis minor
in this illustration. It’s going to be attachedto ribs 3, 4 and 5 for its more proximal attachments
or its origin. And then its distal attachmentor insertion is going to be onto the coracoid
process of the scapula. The pectoralis minoris innervated by the medial pectoral nerve
and there are two main actions of the pectoralisminor. Imagine if it was to shorten, it would
pull the coracoid process downwards therebydepressing the angle of your shoulder, bringing
it down. Another function of the pectoralisminor would be to pull the scapula forward
thereby bringing the point of the shoulderinto a more anterior place. That is referred
to as protrusion of the scapula.
05:28
The serratus anterior is a major muscle on
the lateral side of the thoracic wall andwe can see several of the slips from the serratus
anterior taking their origin from ribs 1 through9. The serratus anterior will wrap around
the margin of your thoracic wall and thenattach to the anterior surface of the scapula
along its medial border.
06:04
The serratus anterior is innervated by the
long thoracic nerve and the actions of theserratus anterior are as follows. One action
is to keep the scapula up against the thoracicwall, which is very important if you’re trying
to push on a heavy object to move it forward.
06:26
It will keep the scapula from winging out
posteriorly.
06:31
Another action of the serratus anterior is
to produce abduction of the humerus abovethe horizontal. So, as you move your humerus
superiorly away from the midline of your bodyand you reach the horizontal, you have to
rotate your scapula so that it rotates upsuperiorly to get your humerus to move above
that horizontal. And if the serratus anterioris paralyzed, there is difficulty in being
able to abduct your humerus above the horizontalposition. The trapezius is helpful in this
movement as well.
07:14
The subclavius is the smallest extrinsic muscle
of the thorax. It is shown in this illustrationattached to the inferior surface of the clavicle
and then it attaches to the first rib. Theinnervation of the subclavius is via the nerve
to the subclavius and the subclavius has twofunctions. The first is its most important
function and that is to stabilize the jointor articulation that exists between the medial
end of your clavicle and the manubrium. Theother function is it can help depress the
clavicle thereby depressing the point of theshoulder to some degree.
Now, that we’ve finished our journey throughthe extrinsic muscles of the thorax, let’s
take a look at the intrinsic muscles. Theintrinsic muscles are listed on this slide.
We have the external intercostal muscles,the internal intercostal muscles, innermost
intercostal muscles, the transverse thoracismusculature, subcostal muscles and then we
have these last two muscles, the serratusposterior (superior and inferior) as well
as the levator costarum. These will be dealtwith the lecture on the back because
they’re on the back side of the thoracicwall.
So, we’ll begin with the external intercostalmuscles. We can see their muscle fibers right
in this particular intercostal space and theexternal intercostal muscles do occupy the
external most aspects of the intercostal spaces.
09:05
Their muscle fibers will run down and in,
as visualized here in the illustration. Withineach intercostal space, they will begin posteriorly
where the tubercles of the ribs are located.
09:20
They’ll run, then, anteriorly and laterally
until they get into the view that we see themhere and then your external intercostals will
end right typically where the costal cartilagesof the ribs join the ribs themselves.
And then, from that point, there is a membranethat will run from the costochondral junctions
that we see here to the lateral edge of yoursternum. Their attachments would be to the
inferior border of the rib above to the superiorborder of the rib below. They are innervated
by the intercostal nerves that travel withinthe intercostal spaces. And their actions
are primarily to help with inspiration. So,they’ll help to lift the ribcage up. And
they’re also very, very important in stabilizingthe intercostal spaces during respiratory
movements.
10:23
The internal intercostal muscles will also
occupy the intercostal spaces. The internalintercostal muscles lie immediately deep to
the external intercostals. They will startposteriorly at the costal angles of the ribs
and as they progress within the intercostalspaces, moving laterally and then anteriorly,
they will terminate right along the lateralmargin of the sternum on each side. And then
they will lie immediately deep to the membranethat’s associated with the external intercostal
muscles.
11:04
If we take a look up here, in the first intercostal
space, we can see the termination of yourexternal intercostal in this first space,
with the muscle fibers running down and in.
11:14
And then here, running from that end of the
exterior intercostal musculature to the sternalborder, you can see the fibers of the internal
intercostals running down and outwards. Andso, those are oriented in a different direction
than the externals. The internal intercostalsare also innervated by intercostal nerves
and their actions are to assist with expiration.
11:46
And they will also contribute to the stabilization
of the intercostal spaces during respiratorymovements.
The innermost intercostals are going to bethe deepest intercostal muscle layer. They
are not well discernible in this illustration,but they occupy the intercostal spaces like
the intercostal muscles do. Their fibers aredirected in the same orientation as those
of the internal intercostals. Consequently,they’re also innervated by the intercostal
nerves and they will assist in the same actionsas the internal intercostal muscles.
The transversus thoracis muscle is shown inthis view. It’s on the deep surface of the
anterior thoracic wall or its internal surface.
12:47
Its muscle fibers are going to originate from
the sternum, xiphoid process and the costalcartilages of ribs 4 through 7. And then the
distal or points of insertion will be to ribs2, 3, 4, 5 and 6. There is variability in
these attachments. Innervation is also providedby the intercostal nerves and these muscles
tend to be more active during expiratory movements.
13:19
The subcostal muscles are not seen in this
particular illustration, but they are locatedon the internal aspect of the posterior thoracic
wall. And they would be lying in this generalarea, a little more developed more inferiorly
along the posterior internal thoracic wall.
13:42
They have the same orientation as do the internal
and innermost intercostal muscles, but what’sunique about them, when you visualize them,
is instead of running withinjust one intercostal space, your subcostal
muscles may attach here, cross this rib belowand then attach to the next rib below that.
So, they’re going to span at least two intercostalspaces; also innervated by intercostal nerves
traveling within the relative intercostalspaces. Actions are to most likely to help
depress the ribs which will assist with expiration.

About the Lecture

The lecture Muscles – Thorax Muscles and Thoracic Wall by Craig Canby, PhD is from the course Abdominal Wall.

Included Quiz Questions

Which of the following muscles inserts on the coracoid process?

Pectoralis minor muscle

Pectoralis major muscle

Subclavius muscle

Serratus anterior muscle

Levator costarum muscle

Which intrinsic muscle of the thorax spans more than one intercostal space as it attaches to a rib above and a rib below?

Subcostal muscle

External intercostal muscle

Internal intercostal muscle

Innermost intercostal muscle

Transversus thoracis muscle

Which muscle performs adduction of the arm?

Pectoralis major

Deltoid

Serratus anterior

Subclavius

Intercostal muscles

What is the nerve supply of the pectoralis minor?

Medial pectoral nerve

Thoracic nerves

Lateral pectoral nerve

Axillary nerve

Intercostal nerves

Which is the smallest extrinsic muscle?

Subclavius

Pectoralis major

Pectoralis minor

Serratus anterior

Serratus posterior

What is the major innervation of intrinsic muscles?

Intercostal nerves

Lateral pectoral nerve

Medial pectoral nerve

Axillary nerve

Thoracic nerve

What of the following is an accurate set of extrinsic muscles of the thorax?

Pectoralis major, pectoralis minor, serrtaus anterior, and subclavius

Pectoralis major and pectoralis minor

Serratus anterior and subclavius

Pectoralis major, pectoralis minor, external oblique, and subclavius

Pectoralis minor and serratus anterior

Author of lecture Muscles – Thorax Muscles and Thoracic Wall

Craig Canby, PhD

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